Abstract: An immunochemical indicator reagent capable of indicating the presence of specific antigenic or haptenic material by hemagglutination reaction therewith, comprising the corresponding antibody globulin molecules chemically coupled to erythrocyte carrier particles through a bifunctional chlorine-terminated coupling agent consisting of either diazo blue B or diazo blue B-terminated condensation products of diazo blue B with glycine. The reagent is prepared by first treating the erythrocyte carrier particles with a proteolytic enzyme such as trypsin to provide the erythrocyte surface with additional exposed active hydrogen-containing groups including free amino groups, then fixing the erythrocyte carrier particles with a fixing agent, such as an aldehyde, and thereafter chemically coupling the erythrocyte carrier particles to the antibody globulin molecules by reaction with the bifunctional chlorine-terminated coupling agent.